With the Giants seemingly all but assured of a playoff berth, Tom Coughlin last week refused to answer any questions about that impending achievement.

If his team doesn’t shape up, Coughlin might be forced to deal with inquiries as to how the Giants played their way out of the playoffs.

“For all the things we did have at stake I just don’t think that was our best,” Coughlin said last night.

Continuing their confounding trend of saving their lousiest performances for their own paying customers, the Giants looked nothing like a playoff-worthy outfit as they were thoroughly outplayed by the Redskins in a 22-10 loss at Giants Stadium that featured more despicable play by Eli Manning.

Adding injury to insult, the Giants suffered a devastating loss, as tight end Jeremy Shockey left early in the third quarter with a fractured left fibula that will keep him out the rest of the season and any postseason games the Giants might have.

Shockey was bent forward by linebacker H.B. Blades before Amani Toomer inadvertently rolled into Shockey’s left leg. Shockey will undergo surgery this week.

Incredibly, it is the third season-ending fractured fibula to befall the Giants, as linebacker Mathias Kiwanuka and running back Derrick Ward are on injured reserve with identical injuries.

“That’s going to be one to the jaw we got to take,” running back Brandon Jacobs said.

“To lose Shockey is definitely going to hurt. Not only is he an emotional leader, he makes plays for us,” added Michael Strahan. “I know as much of a competitor he is it’s really going to kill him to watch.”

Just what will Shockey be forced to watch? Will it be a delayed but inevitable playoff clinching or a collapse?

At 9-5, the Giants still control their own destiny and can secure a playoff spot Sunday with a victory at Buffalo. If the Giants don’t beat the Bills, they might have to beat what likely will be an undefeated Patriots juggernaut in the regular-season finale to clinch a playoff berth. Good luck with that.

“Forget about the playoffs,” right tackle Kareem McKenzie said. “Right now it’s about playing better football. No way, shape or form we should be performing like this.”

The Giants, now a dreary 3-4 at home, followed Manning’s lead and wallowed in badness. They never led and rarely looked in synch. Playing amid swirling, gusting wind, Manning’s receivers dropped a ghastly 12 passes, a dirty dozen that included five flubs by Jacobs, who might have been wearing oven mitts the way he battered the ball.

“I’m very disappointed; the wind wasn’t the best. You just have to concentrate extra hard, and that I didn’t do,” Jacobs said. “Some of ’em I was able to get my back hand on to them, wasn’t able to bring it across. Once you touch it, you should bring it in. I should have had more concentration on the ball.”

Manning was a dismal 6 of 21 for 51 yards in the first half, and when his targets weren’t dropping the ball, he was often missing them. He finished 18 of 52 for 184 yards and one touchdown.

Jacobs ran for 130 yards, but despite the heavy wind the Giants, for some reason, quickly steered away from the ground game.

“It’s just back to the drawing board,” Coughlin said of Manning’s recent struggles. “There obviously isn’t a simple answer.”

Somehow, the Giants made a winner of Todd Collins, a career backup who was making his first start at quarterback in 10 years and didn’t exactly light it up. Collins completed just eight passes in 25 attempts but made just enough plays and more than Manning.

The Skins, led by Clinton Portis (25-126) ran for 153 yards, the most allowed all season by the Giants defense.

Trailing 16-3 at halftime, the Giants looked to make it quitting time as their defense failed to show up to start the second half, allowing the Skins to ramble for a barely contested scoring romp, as Portis ran in to make it 22-3.

Manning finally broke through with a 19-yard touchdown pass to rookie tight end Kevin Boss with 4:37 left in the third quarter. The Giants missed a chance to pull closer when Lawrence Tynes early in the fourth quarter missed a 38-yard field goal attempt.

“We had an opportunity to do it at home and make things a little easy on ourselves,” Strahan said, “and we never seem to do that.”